In an effort to reduce space requirements and weight, eliminate corrosion and improve permanent set resistance as well as fatigue life, it has been proposed that fiber reinforced plastic with its higher strength-to-weight ratio be used as a replacement for steel in torsion bars used in motor vehicle suspension systems. However, several major problems exist when a conventional bundle of fiber strands coated with plastic resin is used to make a replacement torsion bar as contrasted with a leaf spring. For example, the fiber strands are readily adapted in a straight lay arrangement to resist the bending stresses in tension in a leaf spring configuration. But in the case of a torsion bar, there is a torsional component that would be resisted by just the strength of the plastic when straight laid resin impregnated fiber strands are used in their construction. Fiber reinforced plastic is not normally a homogenous material like steel And in the case of straight laid fiber strands, it effectively has only the much lower strength of the plastic resin to resist the torsional component on the bundle of strands when shaped and cured in the form of a straight bar. Moreover, because fiber reinforced plastic has a much lower modulus of elasticity than steel, a cross section of the former must normally be larger in diameter than the steel bar for equivalent strength. It was thought that the resin strength problem could be overcome if the resin impregnated fibers were braided at .+-.45.degree. angle into a tubular cross section and then laid straight to form a torsion bar; the premise here being that the cross section would act more like a homogeneous material. However, this presents substantial processing problems while still requiring a much larger cross section than steel to obtain equivalent strength. A better solution is to form the torsion bar from twisted strands of resin coated fibers that have been laid in a rope forming manner in a lay direction so that the strands always tend to wind further on themselves in reaction to the torsional loading, i.e., unidirectional torsional loading. However, while this approach significantly reduces the required cross section, the diameter-to-length ratio can then become a limiting factor as there is a ratio below which spiral buckling rather than the shear or tensile strength of the fibers determines the load limit.
It is known that the buckling problem could be solved in a very simple low cost manner by the addition of a thin anti-buckling tube about the twisted rope form of torsion bar. In such an arrangement, the anti-bucklng tube extends to near the ends of the torsion bar leaving the latter free for connection in a suspension system, the tube itself not being rigidly attached in any way to the torsion bar or any other suspension component. To the contrary, there is provided a small radial clearance between the torsion bar and tube to assure that it is not a factor in the torque transmission. It was found that the amount of side force on the torsion bar required to prevent its spiral buckling is very small compared to the torsional force that can be withstood and this factor together with no torsional load requirement allows the tube to have a very thin wall.
With the addition of such a thin wall tube to a twisted rope form of torsion bar having a diameter-to-length ratio normally too low for a certain load application, any tendency to buckle by spiraling at what would otherwise be an overload is prevented by the surrounding constraint of the tube interior. That is, the tube holds the twisted rope torsion bar essentially straight while not allowing the tube to enter into the torque transmission because of slippage between the outer helical rope surface and the inner cylindrical tube surface.
Thus, the addition of the anti-buckling tube allows the use of a twisted rope torsion bar with a diameter-to-length ratio substantially smaller than that normally required. As a result, the anti-buckling tube offers the advantage of improved material efficiency (less material required) for a twisted rope torsion bar. Moreover, because the anti-buckling loads are very small and no torque transmission is carried by the anti-buckling tube, the tube itself can be made of low strength material (metal or plastic).